Cataracts are a major cause of blindness in the world and the most prevalent ocular disease. Visual disability from cataracts accounts for more than 8 million physician office visits per year. When the disability from cataracts affects or alters an individual's activities of daily living, surgical lens removal with intraocular lens (IOL) implantation is the preferred method of treating the related visual limitations. In the United States, about 2.5 million cataract surgical procedures are performed annually, making it the most common surgery for Americans over the age of 65. With about 97 percent of cataract surgery patients receiving intraocular lens implants each year, the annual costs for cataract surgery and associated care in the United States is larger than $4 billion.
A cataract is defined as an opacity of a patient's lens, whether it is a localized opacity or a diffuse general loss of transparency. To be clinically significant, however, the cataract must cause a significant reduction in visual acuity or a functional impairment. A cataract occurs as a result of aging or secondary to hereditary factors, trauma, inflammation, metabolic or nutritional disorders, or radiation. Age related cataract conditions are the most common.
In treating a cataract, the surgeon removes the crystalline lens matrix from the lens capsule and replaces it with an intraocular lens (“IOL”) implant. The typical IOL provides a selected focal length that allows the patient to have fairly good distance vision. After cataract surgery, however, the patient typically needs glasses for reading. This is explained by the imaging properties of the human eye, which are facilitated by several optical interfaces.
A healthy youthful human eye has a total power of approximately 59 diopters, with the anterior surface of the cornea (e.g. the exterior surface, including the tear layer) providing about 48 diopters of power, while the posterior surface provides about −4 diopters. The crystalline lens, which is situated posterior of the pupil in a transparent elastic capsule supported by the ciliary muscles, provides about 15 diopters of power, and also performs the critical function of focusing images upon the retina. This focusing ability, referred to as “accommodation,” enables imaging of objects at various distances.
The power of the lens in a youthful eye can be adjusted from 15 diopters to about 29 diopters by adjusting the shape of the lens from a moderately convex shape to a highly convex shape. The mechanism generally accepted to cause this adjustment is that ciliary muscles supporting the capsule (and the lens contained therein) move between a relaxed state (corresponding to the moderately convex shape) and a contracted state (corresponding to the highly convex shape). Because the lens itself is composed of viscous, gelatinous transparent fibers, arranged in an “onion-like” layered structure, forces applied to the capsule by the ciliary muscles cause the lens to change shape.
Isolated from the eye, the relaxed capsule and lens take on a spherical shape. Within the eye, however, the capsule is connected around its circumference by approximately 70 tiny ligament fibers to the ciliary muscles, which in turn are attached to an inner surface of the eyeball. The ciliary muscles that support the lens and capsule therefore are believed to act in a sphincter-muscular mode. Accordingly, when the ciliary muscles are relaxed, the capsule and lens are pulled about the circumference to a larger diameter, thereby flattening the lens, whereas when the ciliary muscles are contracted the lens and capsule relax somewhat and assume a smaller diameter that approaches a more spherical shape.
As noted above, the youthful eye has approximately 14 diopters of accommodation. As a person ages, the lens hardens and becomes less elastic, so that by about age 45-50, accommodation is reduced to about 2 diopters. At a later age the lens may be considered to be non-accommodating, a condition known as “presbyopia”. Because the imaging distance is fixed, presbyopia typically entails the need for bi-focals to facilitate near and far vision.
Apart from the age-related loss of accommodation ability, such loss also has affected IOLs for the treatment of cataracts. Although the research directed at accommodating IOLs has met with some success, the relative complexity of the methods and apparatus developed to date have prevented widespread commercialization of such devices.
IOLs were made from rigid polymeric materials such as polymethyl methacrylate (PMMA), however more resilient polymeric materials are becoming increasingly more popular.
An IOL typically comprises an optic portion providing for the transmission of light, and a haptic portion extending peripherally from the optic portion.
In some IOL designs, such as those described in U.S. Pat. No. 4,932,966 to Christie et al., the optic portion of the lens comprises a thin membrane sealed along its edges to a thicker substrate to form a cavity between the two. Deformable haptics are attached to the periphery of the optic portion and are filled with a driving fluid, such a silicone oil. Movement of the ciliary muscles deforms the haptics and drives fluid from the haptics into the cavity to deflect the thin membrane of the optic portion, thereby modifying the optical power of the lens. As described in that patent, the substrate of the lens may comprise polymethyl methacrylate (PMMA), while the thin membrane and haptics may comprise a silicone elastomer.
U.S. Pat. No. 6,730,123 to Klopotek also describes a fluid-driven IOL, in which an optical fluid is transferred between an optical chamber having a flexible deformable surface and a reservoir to change the optical power of the lens. In the IOL described in that patent, the substrate, deformable surface and internal components are all formed from an acrylic elastomer.
These and other designs suffer from a number of drawbacks. For example, it is known that silicones have high permeability in silicone elastomer materials. Thus, over time, the silicones used in the above-described systems will have a tendency to diffuse through the silicone elastomers. Such diffusion results in a loss of fluid from the IOL and also effects the mechanical properties of the silicone elastomer component, thereby degrading performance of the IOL over time. In addition, silicone elastomer components are likely to permit aqueous fluids from the eye to diffuse into the IOL, further degrading performance of the IOL.
One previously known method to control diffusion of fluids into solid materials involves the deposition of a diffusion barrier on a surface of the lens. For example, U.S. Pat. No. 6,827,966 to Qiu et al. discloses diffusion-controllable coatings on ophthalmic lenses capable of controlling the out-diffusion or release of guest materials from the lenses. The coatings appear better suited for non-implantable lenses than for intraocular lenses, and are applied over the existing lens surfaces rather than being included in the polymeric composition of the lens.
A different process for permanently altering the surface properties of a lens surface, including an IOL, is the Langmuir-Blodgett deposition, described in U.S. Pat. Nos. 4,941,997, 4,973,429, and 5,068,318. Other known processes include controlled spin casting, chemisorptions, and vapor deposition.
A more recent technique developed for coating substrates is a layer-by-layer (“LbL”) polymer absorption process. In particular, U.S. Pat. Nos. 5,518,767 and 5,536,573 to Rubner et al. describe methods of producing bilayers of p-type doped electrically conductive polycationic polymers and polyanions or water-soluble, non-ionic polymers on glass substrates.
U.S. Pat. No. 5,208,111 to Decher et al. describes a method for applying one or more layers to a support modified by the applications of ions and ionizable compounds of the same charges over the entire area. The one or more layers are made of organic materials, which in each layer contain ions of the same charge, the ions of the first layer having the opposite charge of the modified support and, in the case of several layers, each further layer having again the opposite charge of the previous layer.
U.S. Pat. No. 5,700,559 to Sheu et al. discloses a method for making a hydrophilic article having a substrate, an ionic polymeric layer bonded directly onto the substrate, and a disordered polyelectrolyte coating ionically bonded to the ionic polymeric layer. The ionic polymeric layer is obtained by a plasma treatment, an electron beam treatment, a corona discharge, an X-ray treatment, or an acid/base chemical modification of the substrate.
Although each of the above described surface modification techniques are effective for producing an IOL with an altered surface that is different from the remainder of the device, each of these processes requires complex and time-consuming pretreatments of the substrate surface to obtain a highly charged surface.
In addition, the above-described surface modifications involve applying coatings to the outer surfaces of the IOL, rather than providing an IOL with bulk properties that reduces diffusion of fluids into or out of the lens.
In view of the foregoing, it would be desirable to provide an ophthalmic device, such as an IOL, in which the bulk polymeric material provides enhanced resistance to diffusion of fluids into or out of the device.
In addition, it would be desirable that the polymer have properties that would allow it to be deformed to a delivery configuration to enable its implantation in the eye, yet return to a pre-implantation configuration after being implanted in the eye. In addition, it would be desirable that the polymeric composition have a sufficient high refractive index.